Using Unity 2018.2.5f1 + Vuforia 7.5.26
On some devices, the camera feed doesn't fill the screen, resulting in black borders appearing in the top and/or on both sides. In those zones, 3D elements are correctly displayed but there is no camera feed behind them.
See attached screenshot.
I had to remove our UI visual but you can still see the issue. The issue was especially visible on iPhone XR where both sides of the screen and the top were black.